Most corrective lenses for glasses are nowadays made from injectable thermo-plastic or polymerizable, resin-based materials, and are obtained from molded or injected lens blanks. This lens blank presents an exterior face, which is generally spherical or progressive, for which the radius of curvature is determined by the curvature of the frame. At this time, one can observe a marked tendency to favor strongly curved glasses with a large dihedral, wrapping around the curvature of the face.
The correction of each lens is obtained by machining the inner face of the lens blank, depending on the characteristics of the vision to be corrected.
A problem arises with strongly curved glasses having a large dihedral, since, particularly for myopic corrections, the peripheral thickness of the lens is thicker than its thickness at the optical axis level. This extra thickness must be set out in the area where the lenses clip into the frames. It is therefore possible to create a form adapted around the periphery of the lens, in order to snap it into a complementary groove made in the frame. One understands that when the extra, peripheral thickness of the lens is very large, the rim of the lens is visible from a distance, which may look unaesthetic.
Furthermore, those who wear corrective glasses, especially sunglasses, generally appreciate that one shall not be able to distinguish sunglasses with corrective lenses from sunglasses with non-corrective lenses.
It has also been suggested to reduce the extra, peripheral thickness of a corrective lens by making a tier of uniform thickness, which is then clipped into the frame groove. Realizing tiers of this nature is relatively complex and most frequently requires manual grinding or machining operations, which are incompatible with mass production and usually do not comply with quality criteria, particularly as concerns the repeatability of these operations.
Document FR 2 835 931 discloses a method consisting in machining corrective lenses using a particular lens blank having a molded recess in the central portion of its inner face, so as to define the future corrective lens. After creating this lens blank, it is necessary to proceed with a machining operation, which ensures the desired correction. Polishing operations then follow this machining operation. More precisely, the polishing is accomplished with a rotating tool, which the face located in contact with the surface to be polished is of a nearly analogous radius of curvature, and is equipped with an appropriate abrasive coating. One then proceeds to cut through the thin zone separating the future lens from the rest of the lens blank, particularly constituting its peripheral crown.
Considering that the future lens forms an extended edge on the inner face of the lens blank, one observes slope breaks forming relatively protruding ridges. These ridges might lead to problems during machining then polishing operations, which may notably damage the polishing tools as well as the lens.
Indeed, one can observe a degrading of the abrasive polishing coating due to damages caused by the ridges of the relief contour of the future corrective lens.
One problem, which the invention attempts to solve, is that of keeping the advantages procured by the fact that the contour of the corrective lens be made at the same time as the lens blank, but remaining all the while compatible with the use of classical polishing tools.